The Ecology of Teleost Fish Visual Pigments: a Good Example of Sensory Adaptation to the Environment?
The aquatic environment offers a natural laboratory for the study of visual ecology. The colour of natural bodies of water varies from the brown/reds of some freshwater lakes, through the greens of lakes and coastal waters, to the blues of the deep oceans
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Abstract The aquatic environment offers a natural laboratory for the study of visual ecology. The colour of natural bodies ofwater varies from the brown/reds of some freshwater lakes, through the greens oflakes and coastal waters, to the blues of the deep oceans. Teleosts have adapted the visual pigments oftheir rods and cones to take advantage of these different photic environments. Many shallow-living fish are probably tetrachromatic, with sensitivity extending from the near UV to the far-red and utilize the full broad daylight spectrum. Teleosts living in more green waters tend to be blue/green dichromats, having lost sensitivity to the longer and shorter wavelengths. In contrast, deep-sea teleosts generally have pure rod retinae, maximally sensitive to the dim downwelling monochromatic blue light of the ocean. In addition, their rod pigments may be spectrally tuned to be sensitive to their own bioluminescence, which in some cases may be deep red. Many fish probably modify their visual pigment complement, either during development or seasonally, as they change factors such as their feeding habits, geographical location, depth of habitat and photic environment. Key words Visual pigment, retina, teleost, visual ecology, bioluminescence
1 Introduction The lens and cornea of the eye produce an optical representation of the environment on the retina, where image-forming photons are absorbed by visual pigments within the photoreceptors, which convert this physical representation of the world into neurobiological activity. These electrical signals are then processed by the remaining neural cells of the retina and by the cells of the central visual pathway eventually resulting in either conscious perception of the visual stimulus or a reflex response, such as pupil closure or accommodation. The span of wavelengths an animal's visual system is able to respond to is limited partially by the wavelengths transmitted through the ocular media, which often remove short wavelengths (Douglas and Marshall 1999), but primarily by the complement of visual pigments within its photoreceptors. F. G. Barth et al. (eds.), Ecology of Sensing © Springer-Verlag Berlin Heidelberg 2001
Ron H. Douglas
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The sensitivity of a visual pigment is expressed most readily by its absorption spectrum, either determined by microspectrophotometry of single photoreceptors (Bowmaker 1984), or using spectrophotometry of retinal extracts (Knowles and Dartnall 1977), or wholemounts of excised retinae (Douglas et al. 1995). All visual pigments have a bell-shaped absorption spectrum with a point of maximum absorption, the f.. max. at which point they are most likely to absorb a photon and hence convey maximum sensitivity (Fig. 1).
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Fig. 1. Visual pigment templates for a theoretical rhodopsin with A. max 500 nm and a porphyropsin based on the same opsin (A. max 530 nm) forming a pigment pair. The relationship used to cal
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